My initial hypothesis is that these are records ineligible for matching because of a dearth of information needed for matching. But then why are some of these coded as deceased (MORTSTAt = 1)?
The note on the Comparability Tab should sort this out… In 1986, the NCHS did not adjust the original NHIS person weight to control for selection bias, so that the 1986 value of MORTWT is identical to the 1986 value of person weight (PERWEIGHT).